Thin film transistor substrate and method for producing same

ABSTRACT

Each of the auxiliary capacitors ( 6   a ) includes a capacitor line ( 11   b ) comprised of the same material as the gate electrode ( 11   a ) and provided in the same layer as the gate electrode ( 11   a ), the gate insulating film ( 12 ) provided so as to cover the capacitor line ( 11   a ), a capacitor intermediate layer ( 13   c ) provided using the oxide semiconductor and provided on the gate insulating film ( 12 ) so as to overlap the capacitor line ( 11   b ), and a capacitor electrode ( 15   b ) provided on the capacitor intermediate layer ( 13   c ), and the capacitor intermediate layer ( 13   c ) is conductive.

TECHNICAL FIELD

The present invention relates to thin film transistor substrates and manufacturing methods thereof, and more particularly to thin film transistor substrates having auxiliary capacitors and manufacturing methods thereof.

BACKGROUND ART

Active matrix liquid crystal display panels include a TFT substrate having, e.g., a thin film transistor (hereinafter also referred to as the “TFT”) provided as a switching element in each pixel as a minimum unit of an image, a counter substrate placed so as to face the TFT substrate, and a liquid crystal layer enclosed between the substrates. In this TFT substrate, an auxiliary capacitor is provided in each pixel in order to stably retain charge of the liquid crystal layer in each pixel, namely a liquid crystal capacitor.

For example, Patent Document 1 discloses a method for manufacturing a TFT substrate having storage capacitors (corresponding to the auxiliary capacitors) by using four masks. Each storage capacitor is formed by: a stacked pattern of a semiconductor pattern comprised of a semiconductor such as amorphous silicon, a contact layer pattern comprised of amorphous silicon heavily doped with N-type impurities such as phosphorus, etc., and a storage capacitor conductive pattern comprised of a conductive substance such as Mo or a MoW alloy, Cr, Al or an Al alloy, or Ta; a storage electrode provided so as to be located below the stacked pattern, and comprised of a conductive substance such as Mo or a MoW alloy, Cr, Al or an Al alloy, or Ta; and a gate insulating film provided between the stacked pattern and the storage electrode.

CITATION LIST Patent Document

PATENT DOCUMENT 1: Japanese Patent No. 3756363

SUMMARY OF THE INVENTION Technical Problem

However, as disclosed in Patent Document 1, if a semiconductor layer is stacked on one of a pair of electrodes of each auxiliary capacitor in the TFT substrate having the auxiliary capacitors, electric capacitance of the auxiliary capacitor is varied by a voltage that is applied between the pair of electrodes. This decreases display quality of liquid crystal display panels including such a TFT substrate.

The present invention was developed in view of the above problem, and it is an object of the present invention to suppress variation in electric capacitance of an auxiliary capacitor due to semiconductor.

Solution to the Problem

In order to achieve the above object, according to the present invention, a capacitor intermediate layer provided using an oxide semiconductor is made conductive.

Specifically, a thin film transistor substrate according to the present invention includes: a plurality of pixel electrodes arranged in a matrix pattern; a plurality of thin film transistors respectively provided for the pixels electrodes and respectively connected to the pixel electrodes; and a plurality of auxiliary capacitors respectively provided for the pixel electrodes, wherein each of the thin film transistors includes a gate electrode provided on a substrate, a gate insulating film provided so as to cover the gate electrode, a semiconductor layer comprised of an oxide semiconductor and having a channel region provided on the gate insulating film so as to overlap the gate electrode, and a source electrode and a drain electrode which are provided on the semiconductor layer so as to be separated from each other with the channel region interposed therebetween, each of the auxiliary capacitors includes a capacitor line comprised of a same material as the gate electrode and provided in a same layer as the gate electrode, the gate insulating film provided so as to cover the capacitor line, a capacitor intermediate layer provided using the oxide semiconductor and provided on the gate insulating film so as to overlap the capacitor line, and a capacitor electrode provided on the capacitor intermediate layer, and the capacitor intermediate layer is conductive.

According to the above configuration, in each of the auxiliary capacitors, the capacitor intermediate layer provided in the same layer as the semiconductor layer comprised of an oxide semiconductor and forming the thin film transistor has a conductive property rather than a semiconductor property, the gate insulating film is the only dielectric material that retains charge when a voltage is applied between the capacitor line and the capacitor electrode. Thus, electric capacitance is stabilized (1/C_(auxiliary capacitor)=1/C_(gate insulating film)). On the other hand, in the case where the capacitor intermediate layer is comprised of an oxide semiconductor and thus has a semiconductor property, the gate insulating film and the capacitor intermediate layer having a semiconductor property are the dielectric material that retains charge when a voltage is applied between the capacitor line and the capacitor electrode. Thus, the electric capacitance becomes unstable (1/C_(auxiliary capacitor)=1/C_(oxide semiconductor)+1/C_(gate insulating film)). Since the capacitor intermediate layer is provided by using an oxide semiconductor, but is conductive, variation in electric capacitance of the auxiliary capacitor due to the semiconductor can be suppressed. Moreover, the gate insulating film is the only dielectric material that retains charge when a voltage is applied between the capacitor line and the capacitor electrode. Thus, not only the electric capacitance of the auxiliary capacitor is stabilized, but also the electric capacitance of the auxiliary capacitor is increased.

The capacitor electrode may be a part of the drain electrode.

According to the above configuration, since the capacitor electrode is a part of the drain electrode, the auxiliary capacitor is specifically formed by the stacked structure of the capacitor line, the gate insulating film, the capacitor intermediate layer, and the drain electrode.

A productive film may be provided on the semiconductor layer so as to cover at least the channel region, and the capacitor intermediate layer may be exposed from the protective layer.

According to the above configuration, the protective film is provided on the channel region of the semiconductor layer, and the capacitor intermediate layer is exposed from the protective layer. Thus, even if, e.g., vacuum annealing is performed on the substrate to make the capacitor intermediate layer provided using an oxide semiconductor conductive, the channel region of the semiconductor layer is not made conductive, and the semiconductor property of the channel region is maintained.

The capacitor electrode may be a part of the pixel electrode.

According to the above configuration, since the capacitor electrode is a part of the pixel electrode, the auxiliary capacitor is specifically formed by the stacked structure of the capacitor line, the gate insulating film, the capacitor intermediate layer, and the pixel electrode.

An interlayer insulating film may be provided on each of the thin film transistors, and the capacitor intermediate layer may be exposed from the interlayer insulating film.

According to the above configuration, the interlayer insulating film is provided on each of the thin film transistors, and the capacitor intermediate layer is exposed from the interlayer insulating film. Thus, even if, e.g., vacuum annealing is performed on the substrate to make the capacitor intermediate layer provided using an oxide semiconductor conductive, the semiconductor layer forming the thin film transistor is not made conductive, and the semiconductor property of the semiconductor layer can be maintained.

A method for manufacturing a thin film transistor substrate according to the present invention is a method for manufacturing a thin film transistor substrate including a plurality of pixel electrodes arranged in a matrix pattern, a plurality of thin film transistors respectively provided for the pixels electrodes and respectively connected to the pixel electrodes, and a plurality of auxiliary capacitors respectively provided for the pixel electrodes, wherein each of the thin film transistors includes a gate electrode provided on a substrate, a gate insulating film provided so as to cover the gate electrode, a semiconductor layer comprised of an oxide semiconductor and having a channel region provided on the gate insulating film so as to overlap the gate electrode, and a source electrode and a drain electrode which are provided on the semiconductor layer so as to be separated from each other with the channel region interposed therebetween, each of the auxiliary capacitors includes a capacitor line comprised of a same material as the gate electrode and provided in a same layer as the gate electrode, the gate insulating film provided so as to cover the capacitor line, a capacitor intermediate layer provided using the oxide semiconductor and provided on the gate insulating film so as to overlap the capacitor line, and a capacitor electrode provided on the capacitor intermediate layer, the method including: a first step of forming the gate electrode and the capacitor line on the substrate; a second step of forming the gate insulating film so as to cover the gate electrode and the capacitor line, and then forming on the gate insulating film the semiconductor layer and another semiconductor layer that serves as the capacitor intermediate layer; a third step of forming a protective layer so as to overlap the channel region and to expose the another semiconductor layer, and then making the another semiconductor layer exposed from the protective film conductive by vacuum annealing, thereby forming the capacitor intermediate layer; a fourth step of forming on the semiconductor layer the source electrode and the drain electrode that functions as the capacitor electrode; a fifth step of forming on the source electrode and the drain electrode an interlayer insulating film having a contact hole extending to the drain electrode; and a sixth step of forming the pixel electrode on the interlayer insulating film.

According to the above method, in the second step, both the semiconductor layer, which is comprised of an oxide semiconductor and has the channel region provided so as to overlap the gate electrode, and the another semiconductor layer, which is provided so as to overlap the capacitor line and will serve as the capacitor intermediate layer, are formed on the gate insulating film. Then, in the third step, the another semiconductor layer exposed from the protective film overlapping the channel region is made conductive by vacuum annealing, thereby forming the capacitor intermediate layer from the another semiconductor layer while maintaining the semiconductor property of the semiconductor layer. Thus, in each auxiliary capacitor formed by the stacked structure of the capacitor line, the gate insulating film, the capacitor intermediate layer, and the drain electrode, the capacitor intermediate layer provided in the same layer as the semiconductor layer comprised of an oxide semiconductor and forming the thin film transistor has a conductive property rather than a semiconductor property. Accordingly, the gate insulating film is the only dielectric material that retains charge when a voltage is applied between the capacitor line and the capacitor electrode (the drain electrode). Thus, electric capacitance is stabilized (1/C_(auxiliary capacitor)=1/C_(gate insulating film)). On the other hand, in the case where the capacitor intermediate layer is comprised of an oxide semiconductor and thus has a semiconductor property, the gate insulating film and the capacitor intermediate layer having a semiconductor property are the dielectric material that retains charge when a voltage is applied between the capacitor line and the capacitor electrode. Thus, the electric capacitance becomes unstable (1/C_(auxiliary capacitor)=1/C_(oxide semiconductor)+1/C_(gate insulating film)). Since the capacitor intermediate layer is provided using an oxide semiconductor, but is conductive, variation in electric capacitance of the auxiliary capacitor due to the semiconductor can be suppressed. Moreover, the gate insulating film is the only dielectric material that retains charge when a voltage is applied between the capacitor line and the capacitor electrode (the drain electrode). Thus, not only the electric capacitance of the auxiliary capacitor is stabilized, but also the electric capacitance of the auxiliary capacitor is increased.

A method for manufacturing a thin film transistor substrate according to the present invention is a method for manufacturing a thin film transistor substrate including a plurality of pixel electrodes arranged in a matrix pattern, a plurality of thin film transistors respectively provided for the pixels electrodes and respectively connected to the pixel electrodes, and a plurality of auxiliary capacitors respectively provided for the pixel electrodes, wherein each of the thin film transistors includes a gate electrode provided on a substrate, a gate insulating film provided so as to cover the gate electrode, a semiconductor layer comprised of an oxide semiconductor and having a channel region provided on the gate insulating film so as to overlap the gate electrode, and a source electrode and a drain electrode which are provided on the semiconductor layer so as to be separated from each other with the channel region interposed therebetween, each of the auxiliary capacitors includes a capacitor line comprised of a same material as the gate electrode and provided in a same layer as the gate electrode, the gate insulating film provided so as to cover the capacitor line, a capacitor intermediate layer provided using the oxide semiconductor and provided on the gate insulating film so as to overlap the capacitor line, and a capacitor electrode provided on the capacitor intermediate layer, the method including: a first step of forming the gate electrode and the capacitor line on the substrate; a second step of sequentially forming the gate insulating film, an oxide semiconductor film, and a source metal film so as to cover the gate electrode and the capacitor line, then forming on the source metal film a resist pattern in which a portion corresponding to the source electrode and the drain electrode is relatively thick and a portion corresponding to the channel region and the capacitor intermediate layer is relatively thin, and subsequently, etching the source metal film exposed from the resist pattern and the oxide semiconductor film, and then thinning the resist pattern to remove the relatively thin portion, and etching the source metal film thus exposed, thereby forming the semiconductor layer, the source electrode and the drain electrode, and another semiconductor layer that serves as the capacitor intermediate layer; a third step of forming an interlayer insulating film so as to overlap the channel region of the semiconductor layer and to expose a part of the drain electrode and the another semiconductor layer, and then making the another semiconductor layer exposed from the interlayer insulating film conductive by vacuum annealing, thereby forming the capacitor intermediate layer; and a fourth step of forming on the interlayer insulating film the pixel electrode that functions as the capacitor electrode.

According to the above method, in the second step, both the semiconductor layer, which is comprised of an oxide semiconductor and has the channel region C provided so as to overlap the gate electrode, and the another semiconductor layer, which is provided so as to overlap the capacitor line and will serve as the capacitor intermediate layer, are formed on the gate insulating film. Then, in the third step, the another semiconductor layer exposed from the interlayer insulating film overlapping the channel region is made conductive by vacuum annealing, thereby forming the capacitor intermediate layer from the another semiconductor layer while maintaining the semiconductor property of the semiconductor layer. Thus, in each auxiliary capacitor formed by the stacked structure of the capacitor line, the gate insulating film, the capacitor intermediate layer, and the pixel electrode, the capacitor intermediate layer provided in the same layer as the semiconductor layer comprised of an oxide semiconductor and forming the thin film transistor has a conductive property rather than a semiconductor property. Accordingly, the gate insulating film is the only dielectric material that retains charge when a voltage is applied between the capacitor line and the capacitor electrode. Thus, electric capacitance is stabilized (1/C_(auxiliary capacitor)=1/C_(gate insulating film)). On the other hand, in the case where the capacitor intermediate layer is comprised of an oxide semiconductor and thus has a semiconductor property, the gate insulating film and the capacitor intermediate layer having a semiconductor property are the dielectric material that retains charge when a voltage is applied between the capacitor line and the capacitor electrode. Thus, the electric capacitance becomes unstable (1/C_(auxiliary capacitor)=1/C_(oxide semiconductor)+1/C_(gate insulating film)). Since the capacitor intermediate layer is provided using an oxide semiconductor, but is conductive, variation in electric capacitance of the auxiliary capacitor due to the semiconductor can be suppressed. Moreover, the gate insulating film is the only dielectric material that retains charge when a voltage is applied between the capacitor line and the capacitor electrode. Thus, not only the electric capacitance of the auxiliary capacitor is stabilized, but also the electric capacitance of the auxiliary capacitor is increased. Moreover, the thin film transistor substrate is manufactured by using a total of four photomasks, namely a photomask used in the first step, a photomask (capable of providing halftone exposure) used in the second step, a photomask used in the third step, and a photomask used in the fourth step, manufacturing cost of the thin film transistor substrate is reduced.

Advantages of the Invention

According to the present invention, since the capacitor intermediate layer provided using an oxide semiconductor is conductive, variation in electric capacitance of the auxiliary capacitor due to the semiconductor can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a TFT substrate according to a first embodiment.

FIG. 2 is a cross-sectional view of the TFT substrate taken along line II-II in FIG. 1.

FIGS. 3A-3E are diagrams illustrating in cross section a manufacturing process of the TFT substrate according to the first embodiment.

FIG. 4 is a graph showing TFT characteristics in a first experimental example.

FIG. 5 is a graph showing TFT characteristics in a second experimental example.

FIG. 6 is a graph showing the relation between the annealing temperature and the surface resistivity in a third experimental example.

FIG. 7 is a graph showing the relation between the annealing time and the specific electric resistance in a fourth experimental example.

FIG. 8 is a plan view of a TFT substrate according to a second embodiment.

FIG. 9 is a cross-sectional view of the TFT substrate taken along line IX-IX in FIG. 8.

FIGS. 10A-10E are diagrams illustrating in cross section a manufacturing process of the TFT substrate according to the second embodiment.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The present invention is not limited to the following embodiments.

First Embodiment of Invention

FIGS. 1-7 show a first embodiment of a TFT substrate and a manufacturing method thereof according to the present invention. Specifically, FIG. 1 is a plan view of a TFT substrate 30 a of the present embodiment, and FIG. 2 is a cross-sectional view of the TFT substrate 30 a taken along line II-II in FIG. 1.

As shown in FIGS. 1-2, the TFT substrate 30 a includes: an insulating substrate 10; a plurality of gate lines 11 a provided on the insulating substrate 10 so as to extend parallel to each other; a plurality of capacitor lines 11 b each provided between adjoining ones of the gate lines 11 a and arranged so as to extend parallel to each other; a plurality of source lines 15 a provided so as to extend parallel to each other in a direction perpendicular to the gate lines 11 a; a plurality of TFTs 5 a provided at every intersection of the gate line 11 a and the source line 15 a, i.e., in every pixel as a minimum unit of an image; an interlayer insulating film 16 provided so as to cover the TFTs 5 a; a plurality of pixel electrodes 17 provided in a matrix pattern on the interlayer insulating film 16; and an alignment film (not shown) provided so as to cover the pixel electrodes 17.

As shown in FIGS. 1-2, the TFT 5 a includes a gate electrode (11 a) provided on the insulating substrate 10, a gate insolating film 12 provided so as to cover the gate electrode (11 a), a semiconductor layer 13 a provided on the gate insulating film 12 and having a channel region C placed therein so as to overlap the gate electrode (11 a), a protective film 14 provided so as to cover at least the channel region C of the semiconductor layer 13 a, and a source electrode 15 aa and a drain electrode 15 b which are provided on the semiconductor layer 13 a and placed so as to be separated from each other with the channel region C interposed therebetween.

As shown in FIG. 1, the gate electrode (11 a) is a part of the gate line 11 a.

As shown in FIG. 1, the source electrode 15 aa is a portion of the source line 15 a which protrudes in the lateral direction so as to form an L-shape. As shown in FIGS. 1-2, the source electrode 15 aa is connected to the semiconductor layer 13 a via a contact hole 14 a formed in the protective layer 14.

As shown in FIGS. 1-2, the drain electrode 15 b is connected to the pixel electrode 17 via a contact hole 16 a formed in the interlayer insulating film 16, and is connected to the semiconductor layer 13 a via a contact hole 14 b formed in the protective layer 14. As shown in FIGS. 1-2, the drain electrode 15 b overlaps the capacitor line 11 b with both the gate insulating film 12 and a capacitor intermediate layer 13 c interposed therebetween, thereby forming an auxiliary capacitor 6 a.

The semiconductor layer 13 a is comprised of an In—Ga—Zn—O oxide semiconductor such as, e.g., InGaZnO₄.

The capacitor intermediate layer 13 c is comprised of an In—Ga—Zn—O oxide semiconductor such as, e.g., InGaZnO₄, and as shown in FIG. 2, is provided so that a large part of the capacitor intermediate layer 13 c is exposed from the protective film 14. As shown in FIGS. 1-2, the capacitor intermediate layer 13 c is conductive, and is connected to the drain electrode 15 b via a contact hole 14 c formed in the protective film 14.

The TFT substrate 30 a having the above configuration, together with a counter substrate placed so as to face the TFT substrate 30 a and a liquid crystal layer enclosed therebetween, forms an active matrix liquid crystal display panel.

A method for manufacturing the TFT substrate 30 a of the present embodiment will be described with reference to FIGS. 3A-3E. FIGS. 3A-3E are diagrams corresponding to the cross-sectional view of FIG. 2, illustrating a manufacturing process of the TFT substrate 30 a of the present embodiment. The manufacturing method of the present embodiment includes a first step, a second step, a third step, a fourth step, a fifth step, and a sixth step.

First, a metal film such as, e.g., a titanium film (about 2,000 Å) is formed on an entire insulating substrate 10 such as a glass substrate by a sputtering method, and then the metal film is patterned to form a gate line 11 a, a gate electrode (11 a), and a capacitor line 11 b, as shown in FIG. 3A (the first step).

Next, for example, a silicon nitride film (thickness: about 200 nm to 500 nm), a silicon oxide film (thickness: about 20 nm to 500 nm), etc. are sequentially formed by a chemical vapor deposition (CVD) method on the entire substrate having formed thereon the gate line 11 a, the gate electrode (11 a), and the capacitor line 11 b, thereby forming a gate insulating film 12. Moreover, an In—Ga—Zn—O oxide semiconductor film (thickness: about 200 Å to 500 Å) such as InGaZnO₄ is formed at room temperature by, e.g., a sputtering method or a coating method. Then, the oxide semiconductor film is patterned to form a semiconductor layer 13 a and another semiconductor layer 13 b as shown in FIG. 3B (the second step).

Thereafter, an inorganic insulating film such as, e.g., a silicon oxide film (thickness: about 20 nm to 500 nm) is formed by a CVD method on the entire substrate having formed thereon the semiconductor layer 13 a and the another semiconductor layers 13 b. The inorganic insulating film is patterned to form a protective film 14 having contact holes 14 a, 14 b, 14 c as shown in FIG. 3C. Then, the substrate having the protective film 14 formed thereon is subjected to vacuum annealing (annealing temperature: 250° C. to 350° C., annealing time: 5 minutes to 2 hours, and chamber pressure: 10⁻³ Pa or less) by using an infrared heater, a diffusion furnace, etc., thereby making the another semiconductor layer 13 b conductive and thus forming a capacitor intermediate layer 13 c (the third step). If the annealing temperature exceeds 350° C., the glass substrate (the insulating substrate 10 a) tends to break. If the annealing time is in the range of 5 minutes to 2 hours, TFT characteristics can be obtained with high reproducibility. If the chamber pressure exceeds 10⁻³ Pa, the oxygen concentration in the chamber increases, and oxygen defects are less likely to occur, whereby the another semiconductor layer 13 b is less likely to be made conductive.

Subsequently, for example, a titanium film (about 300 Å) and an aluminum film (about 2,000 Å), a titanium film (about 300 Å) and a copper film (about 2,000 Å), or a titanium film (about 300 Å), an aluminum film (about 2,000 Å), and a titanium film (about 1,000 Å), etc. are sequentially formed by a sputtering method on the entire substrate having the capacitor intermediate layer 13 c formed thereon. Then, the metal stacked layer thus formed is patterned (etched with a mixture of acetic acid, phosphoric acid, and nitric acid, and oxalic acid) to form a source line 15 a, a source electrode 15 aa, and a drain electrode 15 b, as shown in FIG. 3D, whereby a TFT 5 a and an auxiliary capacitor 6 a are formed (the fourth step).

Moreover, an inorganic insulating film such as, e.g., a silicon oxide film (thickness: about 20 nm to 500 nm) is formed by, e.g., a CVD method on the entire substrate having formed thereon the TFT 5 a and the auxiliary capacitor 6 a. The inorganic insulating film is patterned to form an interlayer insulating film 16 having a contact hole 16 a, as shown in FIG. 3E (the fifth step).

Lastly, a transparent conductive film such as, e.g., an indium tin oxide (ITO) film (thickness: about 50 nm to 200 nm) is formed by a sputtering method on the entire substrate having the interlayer insulating film 16 formed thereon. Then, the transparent conductive film is patterned to form a pixel electrode 17 as shown in FIG. 2 (the sixth step).

The TFT substrate 30 a can be manufactured in this manner.

Specific experiments conducted will be described with reference to FIGS. 4-7. FIG. 4 is a graph showing TFT characteristics in a first experimental example, and FIG. 5 is a graph showing TFT characteristics in a second experimental example. FIG. 6 is a graph showing the relation between the annealing temperature and the surface resistivity in a third experimental example. FIG. 7 is a graph showing the relation between the annealing time and the specific electric resistance in a fourth experimental example.

First, in the first experimental example, unlike the manufacturing method of the present embodiment, a TFT substrate was prepared by performing vacuum annealing at 220° C. for 5 minutes, and TFT characteristics of the TFT substrate thus prepared were measured (see FIG. 4).

In the second experimental example, as in the manufacturing method of the present embodiment, a TFT substrate was prepared by performing vacuum annealing at 322° C. for 5 minutes, and TFT characteristics of the TFT substrate thus prepared were measured (see FIG. 5).

The results of the first and second experimental examples show that the another semiconductor layer (the capacitor intermediate layer) exhibits a semiconductor property if the vacuum annealing is performed at a low temperature of 220° C. (see FIG. 4), but the another semiconductor layer (the capacitor intermediate layer) exhibits a conductive property if the vacuum annealing is performed at an appropriate temperature of 322° C. (see FIG. 5).

Next, in the third experimental example, after a semiconductor film comprised of InGaZnO₄ was formed on a glass substrate, the surface resistivity was measured in an early stage before vacuum annealing (see line “a” in FIG. 6), after vacuum annealing at 220° C. for 5 minutes (see line “c” in FIG. 6), and after vacuum annealing at 330° C. for 5 minutes (see line “b” in FIG. 6) by using a measuring device (MCP-HT450 made by Mitsubishi Chemical Analytech Co., Ltd.). The “surface resistivity” (Ω/□, ohm per square) refers to the resistance per unit area, and is also called the “sheet resistance” or simply the “surface resistance.” In FIG. 6, “S₁,” “S₂,” and “S₃” on the abscissa shows the difference in composition ratio of In—Ga—Zn—O of the semiconductor film.

The result of the third experimental example shows that, as shown in FIG. 6, the surface resistivity in the early stage and after the vacuum annealing at 220° C. for 5 minutes is in a range in which TFT characteristics can be obtained (1.0×10⁹Ω/□ to 1.0×10¹³Ω/□), and the surface resistivity after the vacuum annealing at 330° C. for 5 minutes is the surface resistivity like a conductor.

Next, in the fourth experimental example, after a semiconductor film comprised of InGaZnO₄ was formed on a glass substrate, vacuum annealing was performed at 220° C. (see line “a” in FIG. 7) or 350° C. (see line “b” in FIG. 7), and the specific electric resistivity was measured at various annealing times by using a measuring device (MCP-HT450 made by Mitsubishi Chemical Analytech Co., Ltd.).

The result of the fourth experimental result shows that, as shown in FIG. 7, in the case where the annealing temperature is 220° C., the specific electric resistance decreases as the annealing time increases, but in the case where the annealing temperature is 350° C., the specific electric resistance decreases as the annealing time increases, but the specific electric resistance becomes substantially constant when the annealing time exceeds 0.3 hours.

The first to fourth experimental examples confirmed that a semiconductor layer comprised of an oxide semiconductor is made conductive by performing appropriate vacuum annealing.

As described above, according to the TFT substrate 30 a of the present embodiment and the manufacturing method thereof, in the second step, both the semiconductor layer 13 a, which is comprised of an oxide semiconductor and has the channel region C provided so as to overlap the gate electrode (11 a), and the another semiconductor layer 13 b, which is provided so as to overlap the capacitor line 11 b and will serve as the capacitor intermediate layer 13 c, are formed on the gate insulating film 12. Then, in the third step, the another semiconductor layer 13 b exposed from the protective layer 14 overlapping the channel region C is made conductive by vacuum annealing, thereby forming the capacitor intermediate layer 13 c from the another semiconductor layer 13 b while maintaining the semiconductor property of the semiconductor layer 13 a. Thus, in each auxiliary capacitor 6 a formed by the stacked structure of the capacitor line 11 b, the gate insulating film 12, the capacitor intermediate layer 13 c, and the drain electrode 15 b, the capacitor intermediate layer 13 c provided in the same layer as the semiconductor layer 13 a comprised of an oxide semiconductor and forming the TFT 5 a has a conductive property rather than a semiconductor property. Accordingly, the gate insulating film 12 is the only dielectric material that retains charge when a voltage is applied between the capacitor line 11 b and the drain electrode 15 b. Thus, electric capacitance can be stabilized (1/C_(auxiliary capacitor)=1/C_(gate insulating film)). On the other hand, in the case where the capacitor intermediate layer is comprised of an oxide semiconductor and thus has a semiconductor property, the gate insulating film and the capacitor intermediate layer having a semiconductor property are the dielectric material that retains charge when a voltage is applied between the capacitor line and the drain electrode. Thus, the electric capacitance becomes unstable (1/C_(auxiliary capacitor)=1/C_(oxide semiconductor)+1/C_(gate insulating film)). Since the capacitor intermediate layer 13 c is provided using an oxide semiconductor, but is conductive, variation in electric capacitance of the auxiliary capacitor 6 a due to the semiconductor can be suppressed. Moreover, the gate insulating film 12 is the only dielectric material that retains charge when a voltage is applied between the capacitor line 11 b and the drain electrode 15 b. Thus, not only the electric capacitance of the auxiliary capacitor 6 a can be stabilized, but also the electric capacitance of the auxiliary capacitor 6 a can be increased. Moreover, since the TFT substrate 30 a includes the semiconductor layer 13 a comprised of an oxide semiconductor, the TFTs 5 a having satisfactory characteristics such as high mobility, high reliability, and a low off-state current can be implemented.

According to the TFT substrate 30 a of the present embodiment and the manufacturing method thereof, the protective film 14 is provided on the channel region C of the semiconductor layer 13 a, and a large part of the capacitor intermediate layer 13 c is exposed from the protective film 14. Accordingly, even if vacuum annealing is performed on the substrate to make the capacitor intermediate layer 13 c provided using an oxide semiconductor conductive, the channel region C of the semiconductor layer 13 a is not made conductive, and the semiconductor property of the channel region C can be maintained.

Second Embodiment of Invention

FIGS. 8-10 show a second embodiment of a TFT substrate and a manufacturing method thereof according to the present invention. Specifically, FIG. 8 is a plan view of a TFT substrate 30 b of the present embodiment, and FIG. 9 is a cross-sectional view of the TFT substrate 30 b taken along line IX-IX in FIG. 8. In the following embodiment, the same portions as those of FIGS. 1-7 are denoted with the same reference characters, and detailed description thereof will be omitted.

The first embodiment is described with respect to the TFT substrate 30 a in which a capacitor electrode forming the auxiliary capacitor is a part of the drain electrode, and the manufacturing method of the TFT substrate 30 a. The present embodiment is described with respect to a TFT substrate 30 b in which the capacitor electrode is a part of the pixel electrode, and a manufacturing method of the TFT substrate 30 b (using four masks).

As shown in FIGS. 8-9, the TFT substrate 30 b includes: an insulating substrate 10; a plurality of gate lines 21 a provided on the insulating substrate 10 so as to extend parallel to each other; a plurality of capacitor lines 21 b each provided between adjoining ones of the gate lines 21 a and arranged so as to extend parallel to each other; a plurality of source lines 24 a provided so as to extend parallel to each other in a direction perpendicular to the gate lines 21 a; a plurality of TFTs 5 b provided at every intersection of the gate line 21 a and the source line 24 a, i.e., in every pixel as a minimum unit of an image; an interlayer insulating film 25 provided so as to cover the TFTs 5 b; a plurality of pixel electrodes 26 provided in a matrix pattern on the interlayer insulating film 25; and an alignment film (not shown) provided so as to cover the pixel electrodes 26.

As shown in FIGS. 8-9, the TFT 5 b includes a gate electrode (21 a) provided on the insulating substrate 10, a gate insolating film 22 provided so as to cover the gate electrode (21 a), a semiconductor layer 23 a provided on the gate insulating film 22 and having the channel region C placed therein so as to overlap the gate electrode (21 a), and a source electrode 24 aa and a drain electrode 24 b which are provided on the semiconductor layer 23 a and placed so as to be separated from each other with the channel region C interposed therebetween.

As shown in FIG. 8, the gate electrode (21 a) is a part of the gate line 21 a.

As shown in FIG. 8, the source electrode 24 aa is a portion of the source line 24 a which protrudes in the lateral direction so as to form an L-shape.

As shown in FIGS. 8-9, the drain electrode 24 b is connected to the pixel electrode 26 via a contact hole 25 a formed in the interlayer insulating film 25. As shown in FIGS. 8-9, the pixel electrode 26 overlaps the capacitor line 21 b with both the gate insulating film 22 and a capacitor intermediate layer 23 c interposed therebetween, thereby forming an auxiliary capacitor 6 b.

The semiconductor layer 23 a is comprised of an In—Ga—Zn—O oxide semiconductor such as, e.g., InGaZnO₄.

The capacitor intermediate layer 23 c is comprised of an In—Ga—Zn—O oxide semiconductor such as, e.g., InGaZnO₄, and as shown in FIG. 9, is provided so that a large part of the capacitor intermediate layer 23 c is exposed from the interlayer insulating film 25. As shown in FIGS. 8-9, the capacitor intermediate layer 23 c is conductive, and is connected to the pixel electrode 26 via a contact hole 25 b formed in the interlayer insulating film 25.

The TFT substrate 30 b having the above configuration, together with a counter substrate placed so as to face the TFT substrate 30 b and a liquid crystal layer enclosed therebetween, forms an active matrix liquid crystal display panel.

A method for manufacturing the TFT substrate 30 b of the present embodiment will be described with reference to FIGS. 10A-10E. FIGS. 10A-10E are diagrams corresponding to the cross-sectional view of FIG. 9, illustrating a manufacturing process of the TFT substrate 30 b of the present embodiment. The manufacturing method of the present embodiment includes a first step, a second step, a third step, and a fourth step.

First, a metal film such as, e.g., a titanium film (about 2,000 Å) is formed on an entire insulating substrate 10 such as a glass substrate by a sputtering method, and then the metal film is patterned by using photolithography to form a gate line 21 a, a gate electrode (21 a), and a capacitor line 21 b, as shown in FIG. 10A (the first step).

Next, for example, a silicon nitride film (thickness: about 200 nm to 500 nm), a silicon oxide film (thickness: about 20 nm to 500 nm), etc. are sequentially formed by a CVD method on the entire substrate having formed thereon the gate line 21 a, the gate electrode (21 a), and the capacitor line 21 b, thereby forming a gate insulating film 22 (see FIG. 10B). Thereafter, an In—Ga—Zn—O oxide semiconductor film such as InGaZnO₄ (thickness: about 200 Å to 500 Å) is formed at room temperature by, e.g., a sputtering method or a coating method to form an oxide semiconductor film 23 (see FIG. 10B). Moreover, for example, a titanium film (about 300 Å) and an aluminum film (about 2,000 Å), a titanium film (about 300 Å) and a copper film (about 2,000 Å), or a titanium film (about 300 Å), an aluminum film (about 2,000 Å), and a titanium film (about 1,000 Å), etc. are sequentially formed by a sputtering method to form a source metal film 24 (see FIG. 10B). Then, a photosensitive resin film R is applied to the source metal film 24, and the photosensitive resin film R thus applied is exposed via a halftone or gray-tone photomask capable of providing halftone exposure, and then is developed to form a resist pattern Ra in which a portion corresponding to a source line 24 a, a source electrode 24 aa, and a drain electrode 24 b is relatively thick and a portion corresponding to a channel region C and a capacitor intermediate layer 23 c is relatively thin, as shown in FIG. 10B. Thereafter, the source metal film 24 exposed from the resist pattern Ra and the oxide semiconductor film 23 formed below the source metal film 24 are etched, and the resist pattern Ra is thinned by ashing etc. to remove the relatively thin portion of the resist pattern Ra, whereby a resist pattern Rb (see FIG. 10C) is formed. Then, the source metal film 24 exposed from the resist pattern Rb is etched to form a semiconductor layer 23 a, the source line 24 a, the source electrode 24 aa and the drain electrode 24 b, and another semiconductor layer 23 b that will serve as the capacitor intermediate layer 23 c, as shown in FIG. 10C (the second step).

Thereafter, an inorganic insulating film such as, e.g., a silicon oxide film (thickness: about 20 nm to 500 nm) is formed by a CVD method on the entire substrate having formed thereon the semiconductor layer 23 a, the source line 24 a, the source electrode 24 aa, the drain electrode 24 b, and the another semiconductor layer 23 b. The inorganic insulating film is patterned by using photolithography to form an interlayer insulating film 25 having contact holes 25 a, 25 b as shown in FIG. 10D. Then, the substrate having the interlayer insulating film 25 formed thereon is subjected to vacuum annealing by using an infrared heater, a diffusion furnace, etc., thereby making the another semiconductor layer 23 b conductive and thus forming the capacitor intermediate layer 23 c as shown in FIG. 10E (the third step).

Lastly, a transparent conductive film such as, e.g., an ITO film (thickness: about 50 nm to 200 nm) is formed by a sputtering method on the entire substrate having the capacitor intermediate layer 23 c formed thereon. Then, the transparent conductive film is patterned by using photolithography to form a pixel electrode 26 as shown in FIG. 9 (the fourth step).

The TFT substrate 30 b can be manufactured in this manner.

As described above, according to the TFT substrate 30 b of the present embodiment and the manufacturing method thereof, in the second step, both the semiconductor layer 23 a, which is comprised of an oxide semiconductor and has the channel region C provided so as to overlap the gate electrode (21 a), and the another semiconductor layer 23 b, which is provided so as to overlap the capacitor line 21 b and will serve as the capacitor intermediate layer 23 c, are formed on the gate insulating film 22. Then, in the third step, the another semiconductor layer 23 b exposed from the interlayer insulating film 25 overlapping the channel region C is made conductive by vacuum annealing, thereby forming the capacitor intermediate layer 23 c from the another semiconductor layer 23 b while maintaining the semiconductor property of the semiconductor layer 23 a. Thus, in each auxiliary capacitor 6 b formed by the stacked structure of the capacitor line 21 b, the gate insulating film 22, the capacitor intermediate layer 23 c, and the pixel electrode 26, the capacitor intermediate layer 23 c provided in the same layer as the semiconductor layer 23 a comprised of an oxide semiconductor and forming the TFT 5 a has a conductive property rather than a semiconductor property. Accordingly, the gate insulating film 22 is the only dielectric material that retains charge when a voltage is applied between the capacitor line 21 b and the pixel electrode 26. Thus, electric capacitance can be stabilized (1/C_(auxiliary capacitor)=1/C_(gate insulating film)). On the other hand, in the case where the capacitor intermediate layer is comprised of an oxide semiconductor and thus has a semiconductor property, the gate insulating film and the capacitor intermediate layer having a semiconductor property are the dielectric material that retains charge when a voltage is applied between the capacitor line and the pixel electrode. Thus, the electric capacitance becomes unstable (1/C_(auxiliary capacitor)=1/C_(oxide semiconductor)+1/C_(gate insulating film)). Since the capacitor intermediate layer 23 c is provided using an oxide semiconductor, but is conductive, variation in electric capacitance of the auxiliary capacitor 6 b due to the semiconductor can be suppressed. Moreover, the gate insulating film 22 is the only dielectric material that retains charge when a voltage is applied between the capacitor line 21 b and the pixel electrode 26. Thus, not only the electric capacitance of the auxiliary capacitor 6 b can be stabilized, but also the electric capacitance of the auxiliary capacitor 6 b can be increased. Moreover, since the TFT substrate 30 b can be manufactured by using a total of four photomasks, namely a photomask used in the first step, the photomask capable of providing halftone exposure and used in the second step, a photomask used in the third step, and a photomask used in the fourth step, manufacturing cost of the TFT substrate 30 b can be reduced. Since the TFT substrate 30 b includes the semiconductor layer 23 a comprised of an oxide semiconductor, the TFTs 5 b having satisfactory characteristics such as high mobility, high reliability, and a low off-state current can be implemented.

According to the TFT substrate 30 b of the present embodiment and the manufacturing method thereof, the interlayer insulating film 25 is provided on each TFT 5 b, and a large part of the capacitor intermediate layer 23 c is exposed from the interlayer insulating film 25. Accordingly, even if vacuum annealing is performed on the substrate to make the capacitor intermediate layer 23 c provided using an oxide semiconductor conductive, the semiconductor layer 23 a forming the TFT 5 b is not made conductive, and the semiconductor property of the semiconductor layer 23 a can be maintained.

Although the In—Ga—Zn—O oxide semiconductor layer is described in the above embodiments, the present invention is also applicable to, e.g., In—Si—Zn—O, In—Al—Zn—O, Sn—Si—Zn—O, Sn—Al—Zn—O, Sn—Ga—Zn—O, Ga—Si—Zn—O, Ga—Al—Zn—O, In—Cu—Zn—O, Sn—Cu—Zn—O, Zn—O, In—O, and In—Zn—O oxide semiconductor layers etc.

Although the gate line (the gate electrode) and the capacitor line have a single layer structure in the above embodiments, the gate line (the gate electrode) and the capacitor line may have a stacked structure.

Although the source line, the source electrode, and the drain electrode have a stacked structure in the above embodiments, the source line, the source electrode, and the drain electrode may have a single layer structure.

Although the gate insulating film has a stacked structure in the above embodiments, the gate insulating film may have a single layer structure.

Although the protective film and the interlayer insulating film have a single layer structure in the above embodiments, the protective layer and the interlayer insulating film may have a stacked structure.

Although each of the above embodiments is described with respect to the TFT substrate in which the drain electrode is an electrode of the TFT which is connected to the pixel electrode, the present invention is also applicable to a TFT substrate in which the source electrode is an electrode of the TFT which is connected to the pixel electrode.

INDUSTRIAL APPLICABILITY

As described above, the present invention can suppress variation in electric capacitance of the auxiliary capacitor due to semiconductor. Accordingly, the present invention is useful for TFT substrates forming liquid crystal display panels.

DESCRIPTION OF REFERENCE CHARACTERS

-   C Channel Region -   R Resist pattern -   5 a, 5 b TFT -   6 a, 6 b Auxiliary Capacitor -   10 Insulating Substrate -   11 a, 21 a Gate Line (Gate Electrode) -   11 b, 21 b Capacitor Line -   12, 22 Gate Insulating Film -   13 a, 23 a Semiconductor Layer -   13 b, 23 b Another Semiconductor Layer -   13 c, 23 c Capacitor Intermediate Layer -   14 Protective Film -   15 aa, 24 aa Source Electrode -   15 b, 24 b Drain Electrode (Capacitor Electrode) -   17 Pixel Electrode -   23 Oxide Semiconductor Film -   24 Source Metal Film -   25 Interlayer Insulating Film -   25 b Contact Hole -   26 Pixel Electrode (Capacitor Electrode) -   30 a, 30 b TFT substrate 

The invention claimed is:
 1. A method for manufacturing a thin film transistor substrate including a plurality of pixel electrodes arranged in a matrix pattern, a plurality of thin film transistors respectively provided for the pixels electrodes and respectively connected to the pixel electrodes, and a plurality of auxiliary capacitors respectively provided for the pixel electrodes, wherein each of the thin film transistors includes a gate electrode provided on a substrate, a gate insulating film provided so as to cover the gate electrode, a semiconductor layer comprised of an oxide semiconductor and having a channel region provided on the gate insulating film so as to overlap the gate electrode, and a source electrode and a drain electrode which are provided on the semiconductor layer so as to be separated from each other with the channel region interposed therebetween, each of the auxiliary capacitors includes a capacitor line comprised of a same material as the gate electrode and provided in a same layer as the gate electrode, the gate insulating film provided so as to cover the capacitor line, a capacitor intermediate layer provided using the oxide semiconductor and provided on the gate insulating film so as to overlap the capacitor line, and a capacitor electrode provided on the capacitor intermediate layer, the method comprising: a first step of forming the gate electrode and the capacitor line on the substrate; a second step of sequentially forming the gate insulating film, an oxide semiconductor film, and a source metal film so as to cover the gate electrode and the capacitor line, then forming on the source metal film a resist pattern in which a portion corresponding to the source electrode and the drain electrode is relatively thick and a portion corresponding to the channel region and the capacitor intermediate layer is relatively thin, and subsequently, etching the source metal film exposed from the resist pattern and the oxide semiconductor film, and then thinning the resist pattern to remove the relatively thin portion, and etching the source metal film thus exposed, thereby forming the semiconductor layer, the source electrode and the drain electrode, and another semiconductor layer that serves as the capacitor intermediate layer; a third step of forming an interlayer insulating film so as to overlap the channel region of the semiconductor layer and to expose a part of the drain electrode and the another semiconductor layer, and then making the another semiconductor layer exposed from the interlayer insulating film conductive by vacuum annealing, thereby forming the capacitor intermediate layer; and a fourth step of forming on the interlayer insulating film the pixel electrode that functions as the capacitor electrode.
 2. The method of claim 1, wherein the oxide semiconductor is an In—Ga—Zn—O oxide semiconductor layer.
 3. The method of claim 1, wherein in the second step, a silicon nitride film and a silicon oxide film are sequentially formed to cover the gate electrode and the capacitor line, thereby forming the gate insulating film.
 4. The method of claim 1, wherein in the second step, the oxide semiconductor film is formed by a sputtering method or a coating method.
 5. The method of claim 1, wherein in the second step, a titanium film and an aluminum film, a titanium film and a copper film, or a titanium film, an aluminum film, and a titanium film are sequentially formed to cover the oxide semiconductor, thereby forming the metal film.
 6. The method of claim 1, wherein the interlayer insulating film is a silicon oxide film. 